Process for Making Bi-Tapered Reinforcing Fibers

ABSTRACT

Non-fractured, non-fibrillatable short fibers, for reinforcing matrix materials such as concrete, have substantially uniform transverse cross-sectional areas along their length for maximum efficiency in pull-out resistance, and two different tapering characteristics along their lengths. Preferred bi-tapered fibers of the invention have a high modulus of elasticity in the range of 5-250 Gigapascal and are preferably modulated in both tapering dimensions. Matrix materials containing the fibers, as well as a method for making the fibers, are disclosed.

FIELD OF THE INVENTION

The present invention relates to fibers for structurally reinforcingmatrix materials such as concrete, and more particularly tonon-stress-fractured, non-fibrillatable short fibers having asubstantially uniform transverse cross-sectional area while varying intwo tapering dimensions along their length, the fibers preferablymodulating periodically in both tapering dimensions.

BACKGROUND OF THE INVENTION

Mortars and concretes are brittle materials comprising a hydratablecement binder and, in the case of mortar, a fine aggregate such as sand,and, in the case of concrete, a coarse aggregate such as crushed gravelor small stones. If a structure made from mortar or concrete issubjected to stresses that exceed its maximum tensile strength, thencracks can be initiated and propagated in that structure.

The ability of the structure to resist crack initiation is understood interms of its “strength,” which is proportional to the maximum loadsustainable by the structure without cracking. This is measured byassessing the minimum stress load (e.g., the “critical stress intensityfactor”) required to initiate cracking.

On the other hand, the ability of the structure to resist propagation(or widening) of an existing crack is described as “fracture toughness.”Such a property is determined by simultaneously measuring the loadrequired to deform or “deflect” a fiber-reinforced concrete (FRC) beamspecimen at an opened crack, and also by measuring the extent ofdeflection. The fracture toughness is therefore determined by dividingthe area under a load deflection curve (generated from plotting the loadagainst deflection of the FRC specimen) by its cross-sectional area.

Reinforcing fibers designed to increase both strength and fracturetoughness properties are known and discussed in U.S. Pat. No. 6,197,423;U.S. Pat. No. 6,503,625; U.S. Pat. No. 6,265,056; U.S. Pat. No.6,592,790; U.S. Pat. No. 6,596,210; and U.S. Pat. No. 6,773,646, whichare owned by the common assignee hereof. In these patents, Rieder et al.disclosed “micro-diastrophic” polymer fibers having irregular and randomdisplacements of polymer material, stress fractures, and microscopicelevated ridges.

Subsequently, in U.S. Pat. No. 6,529,525; U.S. Pat. No. 6,569,526; U.S.Pat. No. 6,758,897; and U.S. Pat. No. 6,863,969, also owned by thecommon assignee hereof, Rieder et al. disclosed polymer fibers havingimproved strength and fracture toughness while retaining dispersibility.By extruding and slitting a flat polypropylene film, and stretching theslit fibers using an extremely high stretch rate, Rieder et al. achievedfibers having a modulus of up to 20 Gigapascals. By avoiding thestress-fracturing of the aforementioned micro-diastrophic flatteningtechnique, the structural integrity of the fibers could be preserved.

Slit polypropylene reinforcing fibers which provide high strength andfracture toughness in concrete are commercially available from GraceConstruction Products, Cambridge, Mass., under the trade name “STRUX®.”

One of the objectives of the present invention is to employ fibershaving a modulus, at least 5 Gigapascals, and more preferably at least20 Gigapascals and more, for increasing the ability of afiber-reinforced matrix material to resist crack initiation.

Another objective is to provide fibers that enhance the fracturetoughness of the matrix material, its ability to resist deflection orwidening of existing cracks. The present inventors must, in other words,now consider how best to control pull-out resistance of high modulusfiber materials. This property must be considered with respect to thesituation wherein the fibers span across a crack or opening in thematrix material.

In U.S. Pat. No. 4,297,414, Matsumoto disclosed polyethylene fibershaving protrusions. These are made by mixing polyethylene having a meltindex of not more than 0.01 at 190° C. under a load of 2.16 kg withpolyethylene having a melt index of more than 0.01, thereby to obtain amixture having a melt index of 0.01 to 10. This mixture was extrudedunder such conditions as to create a jagged surface, which was thenstretched to generate the surface protrusions. In order to achieve thisextreme melt-fracturing of the surface after the stretching treatment,it was important that “the convexities and concavities of the extrudedproduct should be as sharp and deep as possible” (Col. 3, II. 35-39).While no doubt making it more difficult to pull the fibers out ofconcrete, these distressed protrusions and concavities are believed bythe present inventors to create potential breakage points or distressedfeatures, which could lead to premature breakage of the fibers andlowering the reinforcing efficiency for a certain dosage.

One of the concerns in steel fiber product design has been to increasefiber “pull out” resistance, because this increases the ability of thefiber to defeat crack propagation. In this regard, U.S. Pat. No.3,953,953 of Marsden disclosed fibers having “J”-shaped ends forresisting pull-out from concrete. However, such morphology can createentanglement problems and make the fibers difficult to handle and todisperse uniformly within a wet concrete mix. Also, the J-shaped endsare believed by the present inventors to cause premature breakage at thestress-point caused by the folds of the “J” shape. At column 1, lines54-56, Marsden indicated that the end portion of his fibers are supposedto be larger in cross-section than the smallest cross-section of theshank of the filament or fiber. He preferred that the end portions ofhis fibers be larger in both the longitudinal and transverse planarcross-sections. (See e.g., Col. 1, lines 54-56).

A similar large-end approach was taught in Japanese Patent ApplicationNo. JP06263512A2 of Kajima, who disclosed reinforcing short fibers thatgradually tapered from both ends toward the central part of the fibers.The geometry of the Kajima's fibers, which resemble two slender conicalsections joined at their tops, was designed to allow tensile stress onthe fiber to be dispersed into a concrete or synthetic matrix, such thatthe short fiber is mutually compressed and restricted, thereby resistingcrack openings in the matrix. The intent of Kajima is to distribute loadon the fiber to the matrix such that the load is not concentrated on onepoint, so that propagation of cracking in the matrix is prevented bydistributing the force throughout the matrix.

The present inventors believe that prior art fibers, such as thosedisclosed in Marsden and Kajima, lose reinforcing efficiency, becausesuch fibers will tend to break at a narrowed mid-section. In otherwords, the small waist or smallest diameter will define the maximumload-carrying capacity of the individual fiber.

It follows that at the larger ends of such fibers, an excess of materialin the circumferential diameter provides anchoring of the fiber byradial compression of the fiber ends during a crack-opening event in thesurrounding matrix. However, this excess fiber material at both endsdoes not contribute to the maximum load-carrying capacity of the fiber,due to the fact that the breakage is designed to occur at the smallestdiameter.

The reinforcing performance of the fiber is not, therefore, proportionalto the amount of material used in the fiber.

Kajima's tapered fibers would also be difficult to manufacture. Kajimadoes not describe how one is to manufacture the bi-conical shape, or howsuch a tapered geometry can be manufactured on a continuous basis athigh speed. While it can be surmised that the tapered conical shape canbe made by casting metal or polymer material in a mold, it is doubtfulthat such a process would be practical for high volume applications suchas for reinforcing concrete. If the Kajima fibers were to bemanufactured by altering conditions of extrusion, such as narrowing thedie opening or stretching the extruded polymeric material to decreasethe circular diameter, the surface fracturing sought by Matsumoto mightoccur; this would defeat the purpose of Kajima, which is to distributeforces along the body of the fiber.

“Crimped” polymer fibers are known for increasing pull-out resistancefrom concrete and other matrix materials. For example, a sinusoidalfiber is disclosed in U.S. Pat. No. 5,981,630 of Banthia et al. andillustrated as a waveform having a profile amplitude. One problem ofcrimped fibers, as noted in U.S. Pat. No. 5,985,449 of Dill (SpecialtyFilaments), is that fiber balling (e.g., agglomeration) in concrete isdifficult to avoid. Dill thus taught a bundling technique for aligningthe fibers with each other so as to minimize self-entanglement.

Aside from the difficulty in dispersal, it is believed that crimpingdoes not provide a wholly satisfactory solution to enhancing pull outresistance of the fibers. This is because a crack does not always occurat the longitudinal mid-section of a given fiber. The result is thatcrimped fibers can be pulled out of concrete or other matrix insomething of a “snake-like” fashion.

A novel improved tapered fiber is needed which avoids the foregoingdisadvantages, and which can be manufactured both conveniently andeconomically to achieve high reinforcing efficiency as well ascontrolled pull-out resistance.

SUMMARY OF THE INVENTION

In avoiding the disadvantages of the prior art, the present inventionprovides substantially non-stress-fractured, non-fibrillatable shortreinforcing fibers having, on the one hand, substantially uniformtransverse cross-sectional areas along their lengths; and, on the otherhand, having at least two tapering dimensions along their lengths forresisting pull-out from concrete or other matrix material.

Maximum load-bearing capacity and controlled pull-out resistance offibers are realized by having substantially uniform cross-sectionalareas along the fiber length, by avoiding stress fractures created bymechanical flattening, and by avoiding surface anomalies such asprotrusions or concavities caused by melt-fracture extrusion.

Exemplary bi-tapered fibers preferably have a length of 5-100 mm andmore preferably 10-60 mm per fiber; an aspect ratio in terms of lengthto equivalent diameter of 10-500, and more preferably in the range of25-100; a modulus of elasticity in the range of 5-250 Gigapascals, andmore preferably in the range of 20-100 Gigapascals; a tensile strengthof 400-2,500 Megapascals; and a load carrying capacity of 50-5,000Newtons per fiber.

When employed in matrix material such as concrete, the fibers ideallyprovide a balance between anchoring and pull-out when spanning a crackthat occurs in the matrix material. In the case of cracked concrete, itis preferred that the fibers be designed such that half of the fibersspanning across the crack operate to pull out of the concrete while theother half of the fibers spanning the crack should break entirely at thepoint at which the concrete structure becomes pulled completely apart atthe crack. Maximum energy is thus absorbed, from the time the crackedconcrete begins to deform, until catastrophic failure of the concreteoccurs.

The phrase “substantially uniform transverse cross-sectional area” meansthat the cross-sectional area of the fiber body should not vary by morethan 10%, and more preferably by not more than 5%, along major axis Zwhich is defined by the shank or elongated portion of the fiber body. Auniform cross-sectional area is believed by the inventors to confer thehighest reinforcing efficiency possible (unlike the thin-waistedmidsections of the Marsden and Kajima fibers mentioned in theBackground). Fibers of the present invention are short in that they are5-100 mm in length, and more preferably 10-60 mm in length, and theypreferably have an aspect ratio in terms of length to equivalentdiameter of 10-500.

The phrase “two tapering dimensions” refers to and describes twodifferent tapering behaviors within a given length of fiber. If the bodyor shank portion of the short bi-tapered fiber defines longitudinalmajor axis Z, then minor axes X and Y are defined as perpendicular toaxis Z and to each other, and the transverse cross-sectional profile ofthe fiber increases or diminishes gradually in the directions of minoraxes X and Y, even as the transverse cross-sectional area remainssubstantially uniform, from point to point along major axis Z. It ispreferable to have the fibers straight along axis axis Z. As somebending of the fibers can occur during the processing and cutting of thefibers, which is not expected to diminish the performance, thus it ispossible for the fibers to be bent, curved, twisted, or even crimped(and hence the Z axis will be deemed to have such geometry fordefinitional purposes herein), but again it is preferred that the fiberbody or shank portion be as straight as possible.

Thus, exemplary fibers of the invention comprise a fiber body having twoopposed ends defining therebetween an intermediate elongated body (orshank) portion which is substantially non-stress-fractured andsubstantially non-fibrillatable upon mixing into a matrix material suchas cementitious composites, concrete, shotcrete, mortar, asphalt, orsynthetic polymer, the body portion defining longitudinal major axis Zand comprising (A) a substantially uniform transverse cross-sectionalarea that deviates no more than 10%, and more preferably no more than5%, along the length of the elongated body portion which defineslongitudinal major axis Z; and (B) a transverse cross-sectional profilehaving two tapering dimensions for pull out-resistance, the firsttapering dimension occurring in a first transverse minor axis X that isperpendicular to axis Z, the second tapering dimension occurring in asecond transverse minor axis Y that is perpendicular to both axes X andZ; the first and second tapering dimensions having inverted taperingbehaviors wherein, as one moves from point to point along axis Z, (i)the first tapering dimension along axis X increases as the secondtapering dimension along axis Y decreases, (ii) the first taperingdimension along axis X decreases as the second tapering dimension alongaxis Y increases, and/or (iii) both aforementioned taperingcharacteristics described in (i) and (ii) herein are present.

A number of transverse cross-sectional profiles of the fiber arepossible, including but not limited to a circle, oval, square, triangle,or rectangle shapes, other quadrilateral shapes, and polygonal shapes,and can modulate between any of these shapes. The profile can modulatebetween a first shape having a first X/Y ratio (measurement along Xminor axis divided by measurement along Y minor axis) and a second shapehaving a second X/Y ratio that is different from the first profile.Preferably, the profile of a fiber modulates between first and secondX/Y ratios at least once, and more preferably two to fifteen times. Itmay be possible to have up to thirty modulations depending on the natureof the fiber and matrix materials.

The present invention also pertains to matrix materials, includinghydratable cementitious materials containing the fibers, as well asmethods for making the fibers.

Further advantages and features of the invention are further describedin detail hereinafter.

BRIEF DESCRIPTION OF DRAWINGS

An appreciation of the advantages and benefits of the invention may bemore readily comprehended by considering the following writtendescription of preferred embodiments in conjunction with theaccompanying drawings, wherein

FIG. 1A is an illustration of an exemplary short fiber of the presentinvention viewed along the Y axis (which is perpendicular to theindicated X and Z axes) wherein a first tapering behavior is shownbetween first position Z₁ second position Z₂;

FIG. 1B is an illustration of the fiber of FIG. 1A viewed along the Xaxis (which is perpendicular to the indicated Y and Z axes) wherein asecond tapering behavior is shown between first position Z₁ and secondposition Z₂;

FIG. 1C is an illustrative representation of the fiber shown in FIGS. 1Aand 1B wherein the cross-sectional areas at first position Z₁ and secondposition Z₂ are shown overlapping;

FIG. 2A is an illustrative representation of another exemplary fiber ofthe present invention viewed along the Y axis (which is perpendicular tothe indicated X and Z axes) wherein a first tapering behavior is shown;

FIG. 2B is another illustrative representation of the fiber of FIG. 2Aviewed along the X axis (which is perpendicular to the indicated Y and Zaxes) wherein a second tapering behavior is shown;

FIG. 3A is an illustrative representation of another exemplary fiber ofthe present invention viewed along the Y axis (which is perpendicular tothe indicated X and Z axes) wherein a first tapering behavior is shown;

FIG. 3B is another illustrative representation of the fiber of FIG. 3Aviewed along the X axis (which is perpendicular to the indicated Y and Zaxes) wherein a second tapering behavior is shown;

FIG. 4 is an illustrative diagram of an exemplary method of theinvention for making fibers; and

FIG. 5 is an illustration of a fiber material being shaped betweenopposed rollers having undulations in their circumferential surfaces forimparting a gradual tapering or bi-tapering features on the fiber.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned in the Summary above, fibers of the present invention aresubstantially non-stress-fractured and substantially non-fibrillatable.These two fiber characteristics are defined, for example, in U.S. Pat.No. 6,569,526 of Rieder et al., also owned by the common assigneehereof.

By “non-stress-fractured,” the present inventors mean and refer toindividual fiber bodies which are substantially free of internal andexternal stress fractures, such as might be created by grinding methods,mechanical flattening methods (Compare e.g., U.S. Pat. No. 6,197,423 ofRieder et al.), or by extrusion methods which distress the surface ofthe fibers (See e.g., U.S. Pat. No. 4,297,414 of Matsumoto). The generalintent here is to maintain integrity of the individual fiber bodies, notonly in terms of structural fiber integrity, but also integrity anduniformity of the total surface area which contacts the surroundingmatrix material, such that pull-out resistance behavior can be morecarefully controlled by the manufacturer from one fiber batch to thenext.

By “non-fibrillatable,” the present inventors mean and refer toindividual fiber bodies that do not substantially reduce into smallerfiber or fibril units when mechanically mixed into and agitated withinthe matrix composition to the extent necessary to achieve substantiallyuniform dispersal of fibers in the mix. For example, when introducedinto a matrix material such as concrete, which contains sand and stoneaggregates, the fibers of the invention should not reduce into smallerfibrillar bodies during the time required for mixing the fibers suchthat they are evenly dispersed in the concrete.

The term “substantially” is used for modifying the phrases“non-stress-fractured” and “non-fibrillatable” because, duringmanufacture of the fibers or mixing of the fibers into concrete, it ispossible for some surface blemishes or threads to be seen at amicroscopic level, particularly where the fiber are extruded fromsynthetic polymer. Such microscopic phenomena are due to imperfectionsin the polymer or to the extrusion process and/or due to roughening ofthe fiber surface on account of mixing in aggregate-containing concrete.Such microscopic events, then, are not considered manifestations ofstress-fracturing or fibrillation in the sense contemplated by theinventors.

The term “matrix materials” includes a broad range of materials that canbe reinforced by fibers, including adhesives, asphalt, ceramics,composite materials (e.g., resins), plastics, elastomers such as rubber,and structures made from these materials.

Preferred matrix materials of the invention include hydratablecementitious compositions such as paste, mortar, ready-mix concrete,pre-cast concrete, shotcrete, grout, screed, gypsum-based concretes(such as compositions for wall-boards), gypsum- and/or Portlandcement-based fireproofing compositions, waterproofing membranes andcoatings, and other hydratable cementitious compositions, which can besupplied in dry or wet mix form.

Fibers of the invention are used in the paste portion of a hydratablewet “cement,” “mortar,” or “concrete.” These all have pastes which aremixtures composed of a hydratable cementitious binder (usually, but notexclusively, Portland cement, masonry cement, or mortar cement, and mayalso include limestone, hydrated lime, fly ash, granulated blast furnaceslag, pozzolans, and silica fume or other materials commonly used insuch cements) and water. Mortars are pastes additionally including fineaggregate, such as sand. Concretes are mortars additionally includingcoarse aggregate such as small stones and crushed gravel. “Cementitiouscompositions” of the invention thus refer and include all of theforegoing. For example, a cementitious composition may be formed bymixing required amounts of certain materials, such as hydratablecementitious binder, water, and fine and/or coarse aggregates, as may bedesired, with the fibers described herein.

Exemplary fibers of the invention may comprise metal (e.g., steel) orone or more synthetic polymers selected from the group consisting of,but not limited to, polyvinyl alcohol, polyethylene (including highdensity polyethylene, low density polyethylene, and ultra high molecularweight polyethylene), polypropylene, polyoxymethylene, polyamide, andthermotropic liquid crystal polymers. The preferred geometry can also beobtained with a composite material such as a bundle of fine filaments orfibrils bonded together by a suitable resin or inorganic binder to forma thicker “macro-fiber” unit. For example, individual fiber bodies maycomprise continuous fine carbon or aromatic polyamide (commonly known asKEVLAR® material) or glass/ceramic or ultrahigh molecular weightpolyethylene or even metal fiber filaments of very high modulus andstrength bonded together by resins such as nylon, epoxy, polyolefin, andothers, to provide a composite fiber possessing the required modulus andstrength. Other possible materials from which the fibers can be formedinclude metals (e.g., steel, stainless steel), inorganic (e.g.: glass,ceramic)).

The drawings provide illustration of various exemplary fibers tofacilitate comprehension of the present invention. They are presented toemphasize the tapering concept and are not drawn to scale. The fibershave two opposing ends which define therebetween an intermediate body orshank portion which coincides with and defines a longitudinal andcentrally located major axis Z. Thus, if the fiber is bent, curved, ortwisted, it must be physically straightened by hand or held by othermeans so that the fiber can be properly viewed and considered in threedimensional space for purposes of the present invention. One transverseminor axis which is perpendicular to the major axis Z will be designatedthe axis X (and this will usually correspond to the width, or widerdimension, of the fiber); and a second transverse minor axis which isperpendicular to both the minor axis X and longitudinal major axis Zwill be designated as axis Y (and this will usually correspond to thethickness dimension where the fiber has an oval, rectangular, orotherwise flat cross-sectional shape or profile).

The present inventors contemplate that it is possible to have more thantwo different tapering dimensions, and that the at least two taperingdimensions need not necessarily be at 90 degree angles with respect toeach other. It is possible, therefore, to have one tapering dimensionwhich resides at, for example, a 45 degree angle with respect to thesecond tapering dimension. For small fibers as contemplated in thepresent invention, however, it is believed that having an approximately90 degree angle (perpendicular) for the at least two tapering dimensionsis most convenient.

As shown in FIG. 1A, an exemplary short bi-tapered fiber 10 of thepresent invention has a first tapering behavior or characteristic whenviewed along the transverse minor Y axis, which is perpendicular to thetransverse minor X axis and longitudinal major axis Z. The major axis Zcoincides with and is defined by the elongated body portion (or shank)of the fiber 10 between one end, which is designated first position Z₁,and the other end, which is designated as second position Z₂. This firsttapering behavior is seen, when moving the eyes from left to right ofthe illustration, as a gradual narrowing of the width from Z₁ to Z₂along the length of the fiber 10.

FIG. 1B illustrates a second tapering behavior or characteristic of theexemplary bi-tapered fiber illustrated in FIG. 1A when rotated 90degrees about its longitudinal major axis Z. The fiber, as shown in FIG.1B, is viewed along transverse minor axis X, which is perpendicular tothe transverse minor axis Y and longitudinal major axis Z. The secondtapering behavior is seen, when moving the eyes from left to right ofthe illustration, as a gradual thickening from Z₁ to Z₂ along the lengthof the fiber 10.

FIG. 1C is a composite illustration of the transverse cross-sectionalprofiles, taken at first position Z₁ and second position Z₂, of theexemplary bi-tapered fiber 10 illustrated in FIGS. 1 and 2, when viewedalong longitudinal major axis Z. Minor transverse axis X is designatedin the horizontal direction of FIG. 1C, while minor transverse axis Y isdesignated in the vertical direction of FIG. 1C. Hence, thecross-sectional profiles of the exemplary bi-tapered fiber 10 at Z₁ andZ₂ are shown as an overlapping composite illustration. The shadedportions located along the minor X axis and designated as ΔX_(L)(located to the left of the Y axis) and ΔX_(R) (located to the right ofthe Y axis) represent the change in cross-sectional area occurring inthe fiber 10 starting at first location Z₂ and progressing towardssecond location Z₁. The shaded portions located along the minor Y axisand designated as ΔY_(T) (located above the X axis) and ΔY_(B) (locatedbelow the X axis) represent the change in cross-sectional area occurringin the fiber between first fiber location Z₁ and second fiber locationZ₂. The difference between the sum total of ΔX_(L) and ΔX_(R) should beno more than 10%, and more preferably no more than 5%, of the sum totalof ΔY^(T) and ΔY^(B); one of the purposes of the present invention is toachieve fibers having a substantially uniform transverse cross-sectionalarea along longitudinal axis Z.

In further preferred embodiments of the invention, the amount oftapering can be determined in accordance with the followingrelationship:

ΔR/ΔL∝(σ·A)/(ε·n·L_(f))

wherein “ΔR” represents the change between the ratio R at Z₁ and theratio R at Z₂ (wherein R=width/thickness); “ΔL” represents the lengthalong axis Z of one modulation (e.g., the distance between Z₁ and Z₂ inFIGS. 1A and 2A); “σ” represents fiber tensile strength; “A” representsfiber cross-sectional area; “ε” represents the fiber compressivemodulus; “n” represents the number of modulations per fiber length; and“L_(f)” represents the total length from end to end of the individualfiber.

The softer the fiber material, the larger is the tapering or amplitudein the profile dimensions, e.g., along transverse axes X and Y.

Another exemplary bi-tapered fiber 10 of the present invention is shownin FIG. 2A. The fiber 10, which is viewed along transverse minor axis Y,has a gradual widening, when viewed from left to right of theillustration, between first fiber location Z₁ and second fiber locationZ₂, and a gradual narrowing in width between second fiber location Z₂and third fiber location Z₃. The same fiber 10 is shown along transverseminor axis X in FIG. 2B. It has a corresponding gradual decrease inthickness between first location Z₁ and a second location Z₂, and acorresponding gradual increase in thickness between second location Z₂and third location Z₃. A composite illustration of the cross-sectionalprofiles, taken at Z₁, Z₂, and Z₃, when viewed in the direction alongmajor axis Z, may resemble the composite overlapping illustration of theprofiles shown in FIG. 1C. While it is possible that the differentcross-sectional profiles be used for Z₁ and Z₃ could be different, thisis not preferable.

The bi-tapered fiber 10 shown in FIGS. 2A and 2B can be said to have twomodulations (i.e., from Z₁ to Z₂ and from Z₂ to Z₃). The cross-sectionalprofiles or shapes at Z₁ and Z₃ are the same, and it is preferred thatthe distance Z₁ to Z₂ and distance from Z₂ to Z₃ be approximately thesame, such that the fiber 10 is said to have two modulations or onecomplete modulation period or cycle of modulation.

A further exemplary bi-tapered fiber 10 of the present invention isshown in FIGS. 3A and 3B. The fiber 10, which is viewed in FIG. 3A alongtransverse minor axis Y, has a gradual widening from Z₁ to Z₂ and fromZ₃ to Z₄ and a gradual narrowing in width from Z₂ to Z₃ and Z₄ to Z₅.The fiber 10, when viewed along transverse minor axis X in FIG. 3B, hasa corresponding gradual decrease in thickness from Z₁ to Z₂ and from Z₃to Z₄ and a gradual increase in thickness from Z₂ to Z₃ and Z₄ to Z₅.Preferably, the cross-sectional profiles of Z₂ and Z₄ are the same,while the cross-sectional profiles of Z₁, Z₃, and Z₅ are the same, andthe distances between Z₁, Z₂, Z₃, Z₄, and Z₅ are the same, such that theexemplary fiber 10 as shown in FIGS. 3A and 3B can be said to have fourmodulations or two complete modulation periods or cycles of modulation.

While the exemplary fibers of FIGS. 1A-1B, 2A-2B, and 3A-3B were shownwith fiber ends coinciding with extremest cross-sectional profiledimensions (i.e., of largest or least dimensions along axis X or axisY), this was intended for illustrative purposes only, because thepresent inventors do not believe it necessary that the fiber length becut so precisely as to coincide with greatest width portion (X axis) orthinnest portion (Y axis). This will be the case particularly where theindividual fiber bodies have two or more modulations (or cycles ofmodulation).

Accordingly, exemplary fibers can have at least two modulations whereinthe cross-section profile of the fiber increases and decreases in widthcorresponding to a decrease and increase in thickness. Further exemplaryfibers can have at least four modulations wherein the cross-sectionprofile of the fiber twice increases and twice decreases in widthcorresponding, respectively, to two decreases and two increases inthickness.

Due to the unique geometry of the fibers, a single fiber can be obtainedby cutting at any two points along axis Z. Theoretically, there is nominimum (or even a fraction of) or maximum number of modulationsrequired in a single fiber as long as the width/thickness ratios (X/Y)are different on both sides of a reference point on the Z axis of thefiber. Preferred fibers will have at least one modulation and preferablyno more than thirty modulations per fiber body. Thus, if a crack opensin the surround matrix material, more fibers within the batch of fibersused for treating the matrix material, will have a chance to resistcrack opening regardless of whether the crack exists towards the middleor near the ends of the individual fibers.

FIG. 4 is an illustrative diagram of an exemplary method of theinvention for making fibers 10. In the case of synthetic polymer fibers,the polymer material is extruded through a die 20 or other extrusiondevice preferably in a monofilament 8 or sheet form, is cooled 24 suchas by running through a water bath or between chill rolls. If thepolymer is in sheet form, then it should be slit into individual fibersafter this cooling stage. The individual fiber strands are then oriented(or stretched) in the longitudinal direction to increase the (Young's)modulus of elasticity. Optionally, this can be done by drawing thepolymer between opposed rollers 26, through a thermal softening zone 27(using an oven, hot air blower, or other heating device), and thenthrough another set or sets of opposed rollers rotating at a much fasterrate (e.g., 5-10 and more preferably 5-25 times faster than rollers 26).After the optional orienting/stretching stage, the polymer strands 8 areshaped to have a modulating profile by drawing the polymer betweenopposed rollers 30 having outer circumferential surfaces having periodicundulations therein for deforming the polymer and create a taperingfeature in at least one dimension. The present inventors believe that itmay be possible for one set of rollers (such as designated at 30) toinduce tapering modulations in two dimensions (both axis X and axis Ydirections) of the fiber material. Optionally, a second pair of rollers32 having circumferential surface undulations may be employed. Eventhough two opposing rollers having outer circumferential surfaces withperiodic undulations are preferred, it is possible to achieve desiredresults from only one of the rollers having periodic undulations and theother with circular circumference. Alternatively, both rollers can be ofcircular circumference with suitable diameter and one of them can beoscillated with controlled amplitude and frequency in a directionperpendicular to the fiber axis and the other being stationary.

FIG. 5 is an illustration of a fiber material 8 being shaped betweenopposed rollers 30 having undulations 31 on their circumferentialsurfaces for imparting a modulated tapering effect on the fiber 10.(Another set of these rollers having the same or similar undulations canbe employed as the perpendicularly angled rollers 32 shown in FIG. 4).As shown in FIG. 5, the number of undulations 31 per inch (or cm), theamount of curvature of the undulations, the gap between the roller 30surfaces, and the temperature of the fiber (some elevation oftemperature above room temperature is desirable, if the polymer is notheated in the optional orienting/stretching stage) are all within thecontrol of the skilled artisan to achieve the desired gradual tapering.

Again, it may be possible to achieve the bi-tapering effect describedabove merely by using one set of opposed rollers where, for example,displacement of polymer is achieved primarily in one of the transverseaxis directions (e.g., axis X). For example, the undulations 31 of therollers 30 are used to control the thickness tapering (in the directionof Y axis) while the polymer displaced provides the width-wise tapering(in the direction of the X axis). Optionally, another set of opposedrollers 32 as shown in FIG. 4 can be used, this time aligned with theirrotational axes perpendicular to the rotational axes of rollers 30 toachieve an accurate tapering feature in the dimension perpendicular toopposed rollers 32. The curvature of the undulations on thecircumferential surface of this other set of opposed rollers 32 may beslightly different than the undulation curvature on the first set ofopposed rollers 30, in order to maintain a substantially uniformcross-sectional area along the length of the fiber body.

Preferably, each of the rollers used in a set of opposed rollers hasundulations that are evenly spaced around their circumferentialsurfaces, and each of the rollers within a given set of rollers aremechanically linked, such as by gears, such that they rotate at the samerate and such that the undulations of each roller coincide with theundulations of the opposed roller during rotation.

An exemplary process of the present invention therefore comprisescompressing a metal or polymer fiber between opposed rollers havingcircumferential surfaces having matched undulations which periodicallyrepeat in identical patter on each of said opposed rollers, therebycreating a tapering of fiber material in each of two transversedimensions perpendicular to the longitudinal major axis of the fiber.

In a further exemplary process of the present invention for makingfibers comprises extruding a polymeric material to form a monofilamentfiber body, optionally stretching the polymer to orient the material,and shaping the fiber material using a first set of opposed rollershaving circumferential undulations conformed to impress a taperingeffect into the fibers. The opposed rollers can be operative to createtapering effects in each of two transverse dimensions, which areperpendicular to each other (or at least 45 degrees apart) and to thelongitudinal major axis of the fiber. If not, then alternatively asecond set of opposed rollers can be used which are arranged so as tohave their rotational axes perpendicular to the rotational axes of saidfirst set of opposed rollers, so as to achieve a tapering effect in asecond dimension. The rotation and spacing of the rollers and rollersets should be designed such that the cross-sectional area of the fiberremains substantially constant (i.e., ±10% or less) along the fiberlength.

In further exemplary embodiments of the invention, the fibers cancomprise two different sets of individual fibers, wherein a first set offibers comprises the above-described bi-tapered fibers, and a second setof fibers comprises fiber bodies having a different fiber body geometryor body dimensions. For example, the second set of fibers may notcontain bi-tapered bodies, but may otherwise have different geometricalshapes, such as flat ribbon (quadrilateral), round, oval, rectangular,or other cross-sectional profile shapes. The second set of fibers canalso be bi-tapered but having different taper dimensions in terms ofmodulation distance (e.g., between Z₁ and Z₂) or distension (change inwidth or thickness) when compared to the first set of fibers.

Moreover, the fibers of the present invention, though having elongatedbodies (or shank portions), do not necessarily need to be straight,since some bending, curving, twisting, or crimping may naturally occurduring the manufacture or packaging process.

The present invention is not to be limited by the foregoing examples andillustrations, which are provided for illustrative purposes only.

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 27. A process for making fibers, comprising: compressing a metal or polymer fiber between opposed rollers having circumferential surfaces having matched undulations which periodically repeat in identical pattern on each of said opposed rollers, thereby creating a tapering of fiber material in each of two transverse dimensions perpendicular to the longitudinal major axis of the fiber.
 28. The process of claim 27 wherein a fiber is extruded to form a monofilament fiber body, optionally stretching the polymer to orient the material, and thereafter tapering the fiber using a first set of opposed rollers having circumferential undulations conformed to impress a tapering effect into the fibers.
 29. The process of claim 27 further comprising shaping the fiber using a second set of opposed rollers which are arranged so as to have their rotational axes perpendicular to the rotational axes of said first set of opposed rollers.
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 34. A process for making bi-tapered fibers, comprising: extruding polymer through an extrusion device to form monofilament fibers or a polymer sheet, and, if in sheet form, slitting said sheet to form separate fibers; stretching the fibers in the longitudinal direction to increase modulus of elasticity; and drawing the polymer between opposed rollers thereby shaping said fibers, and cutting the fibers into individual lengths; said polymer extruding, stretching, and cutting into individual lengths being accomplished so as to obtain fiber bodies have two opposed ends defining therebetween an intermediate elongated fiber body portion which is substantially non-fractured and non-fibrillatable when mixed into a matrix material such as concrete, shotcrete, mortar, grout, or synthetic polymer, the fiber body portions having a length of 5 mm-100 mm; the body portion defining longitudinal major axis Z and comprising a transverse cross-sectional profile having two tapering dimensions for pull out-resistance from the concrete or other matrix material, the first tapering dimension occurring in a first transverse minor axis X that is perpendicular to axis Z, the second tapering dimension occurring in a second transverse minor axis Y that is perpendicular to both axes X and Z; the first and second tapering dimensions having inverted tapering behaviors wherein, along axis Z, the first tapering dimension along axis X increases as the second tapering dimension along axis Y decreases, and the first tapering dimension along axis X decreases as the second tapering dimension along axis Y increases; the transverse cross-sectional area remaining substantially uniform along axis Z having said inverted tapering dimensional behaviors; and said fibers having an aspect ratio in terms of length to equivalent diameter of not less than 10 and not greater than 500, a modulus of elasticity not less than 5 Gigapascals and not greater than 250 Gigapascals, a tensile strength not less than 400 Megapascals and not greater than 2,500 Megapascals, and a load carrying capacity of not less than 50 Newtons per fiber and not greater than 5,000 Newtons per fiber; said tapering being achieved by (a) at least one of said two opposed rollers having undulations to form said fiber tapering in said fiber bodies or (b) both rollers having circular circumferential dimensions and oscillating at least one of said two opposed rollers in a direction perpendicular to the fiber axis being drawn between said rollers, whereby said fibers acquire said tapering dimensions.
 35. The process of claim 34 wherein two rollers are employed, at least one of said rollers having periodic undulations operative to taper said fiber bodies.
 36. The process of claim 34 wherein both of said rollers have periodic undulations operative to taper said fiber bodies.
 37. The process of claim 34 wherein both of said rollers have circular circumferential dimensions, and one of said rollers is oscillated towards the other and perpendicular to the fiber body.
 38. The process of claim 34 wherein two sets of opposed rollers are employed to achieve said tapering.
 39. The process of claim 34 wherein said opposed rollers have undulations that are evenly spaced around their circumferential surfaces.
 40. A method for modifying a cementitious composition, comprising: combing a cementitious binder, at leas one aggregate, and the fibers made by the process of claim
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